Lens unit cemented at circumference of each of lens elements and optical system including the same

ABSTRACT

A circumferentially cemented lens unit includes first and second lenses opposed to each other with a space therebetween on an optical axis thereof. The first lens has a concave surface, part of the second lens is a contact portion disposed radially inward from the periphery of the second lens and is in contact with a peripheral portion of the concave surface of the first lens. The first and second lenses are cemented to each other at a portion radially outward from the contact portion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lens structure to be incorporated inan optical system and, more particularly, to a circumferentiallycemented lens unit obtained by cementing a plurality of single lenses attheir circumferences, and an optical system incorporating the same.

2. Related Background Art

Optical axes of single lenses are slightly decentered or deviated fromeach other because the optical surface of each single lens is a curvedsurface. For this reason, conventionally, the optical axes of singlelenses must be accurately aligned and fixed to constitute ahigh-precision optical system. That is, centering is generally performedto obtain good optical performance.

An optical system is constituted by a plurality of lens groups. Eachlens group is constituted by combining single lenses havingcharacteristics corresponding to a specific application purpose. Tocombine single lenses to obtain a lens group, a cylindrical lens holderhaving a diameter almost equal to that of each of the single lenses tobe combined is used.

As a general method of housing single lenses constituting a lens groupin a lens holder, the single lenses are inserted into the lens holder inan order of combination, the decentered or deviated states of the singlelenses are adjusted, and the single lenses are then fixed by a pressring or the like.

In this case, the single lenses constituting the optical system areinserted into the lens holder one by one. In this case, a small space isrequired between the lens holder and each single lens because the singlelenses cannot be smoothly inserted into the lens holder unless thediameter of the lens holder is slightly larger than that of each singlelens.

When the single lenses are stored in the lens holder one by one, theopposing optical surfaces of the adjacent lenses may be brought intocontact. Sometimes, the contact portion of these adjacent lenses may bedeviated from an optimal position. For example, as shown in FIG. 5, whenthe optical surface of at least one single lens 102 is a recessed curvedsurface, the adjacent single lens tends to be inclined toward theconcave lens surface, and the contact portion is deviated from theoptimal position.

In insertion of at least two single lenses into a lens holder, assumingthat one lens has a concave lens surface and that this concave lenssurface is brought into contact with the other lens, the contact portionis deviated from the optimal position due to the presence of a spacebetween each single lens and the lens holder. As a result, decenteringoccurs in inserting single lenses into a lens holder.

For example, when a concave lens is inserted into a lens holder as thefirst lens, it can be housed such that the center of the lens holder isaligned with the optical axis of the concave lens. When a second lens103 is inclined toward a first concave lens 102, deviation occurs withrespect to a central axis 104, as shown in FIG. 5 or 6, regardless ofthe type (i.e., convex or concave) of the second lens 103.

This lens inclination poses a serious problem in a lens required for ahigh precision. A cumbersome operation is required for an operator tocorrect this inclination. In some cases, adjustment is impossible, anddecentered single lenses may be fixed in a lens holder.

Decentering is caused by insertion of single lenses into a lens holderin addition to decentering of a single lens itself. In some cases, theoptical axes of single lenses are considerably deviated from each other,and a lens group having excellent optical characteristics cannot beobtained. In particular, when a concave lens surface is to be cementedto another concave lens surface, it is difficult to avoid decentering.In formation of a lens group using a lens holder, high-precisionassembly free from decentering forces the operator to precisely adjusteach single lens, and working efficiency and productivity are degraded.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a lensgroup in which optical axes of single lenses constituting the lens groupare properly aligned, and a lens structure obtained by simply assemblinga lens group.

A circumferentially cemented lens unit includes first and second lensesopposing through a space on an optical axis. The first lens has aconcave surface, part of the second lens is in contact with theperipheral portion of the concave surface of the first lens. The firstand second lenses are cemented to each other at a portion radiallyoutward from the contact portion.

The present invention also provides a lens system including thecircumferentially cemented lens unit described above. This lens systemincludes a plurality of lenses housed in a lens holder such as a lensbarrel. At least two of the plurality of lenses are cemented to form thecircumferentially cemented lens unit.

In the circumferentially cemented lens unit, the plurality of lenses canbe fixed to each other without using any lens holder because the firstand second lenses opposing through the space on the optical axis arecemented at the portion radially outward from the contact portion.Therefore, fixing of the plurality of lenses in the lens holder is freefrom decentering, and a cemented lens can be stably obtained with a highoptical precision.

As described above, the single lenses are cemented to obtain a cementedlens. If necessary, this cemented lens is housed in a lens barrel toobtain a highly precise lens system.

According to the present invention, a plurality of lenses are cementedto obtain a first cemented lens element, and the first cemented lenselement is incorporated in a lens barrel to obtain a finished opticalsystem. The number of lens elements used in assembly of an opticalsystem can be reduced to improve working efficiency.

A special jig or operation is required for cementing single lensesbecause they are cemented utilizing peripheral portions which areoptically unnecessary portions. Assembly and adjustment are simple, andgood workability can be obtained.

According to the present invention, as described above, management costand material cost, as of a lens barrel, can be reduced. At the sametime, a compact optical system can be obtained, and the radial size of aproduct can be reduced. Another new mechanism can also be arranged inthe extra radial space.

The first and second lenses may be cemented continuously along theentire circumferences thereof or at a plurality of circumferentialpositions. In the latter case, the first and second lenses aretemporarily cemented, centering is performed prior to actual mounting ofthe lenses into the lens holder, and the first and second lenses areactually mounted in the lens holder. When the first and second lensesare temporarily cemented, they must be firmly fixed with a press ring orthe like.

More preferably, at least one of the first and second lenses may bechamfered to form a groove open outward with respect to the contactportion, and an adhesive may be filled in the groove. In this case, anadhesive will not swell out from the circumferences of the cementedportions of the lenses to prevent an interference with mounting thecemented lenses into a lens holder. For this reason, the lens surfaceswill not be stained with the swollen adhesive, and the first and secondlenses are firmly cemented.

When a temperature difference between the outer air and the space in thecircumferentially cemented lens unit is increased as in a case wherein auser quickly moves from a cold place to a warm place, the outer lenssurface delicately expands while the inner lens surface is keptunchanged or shrinks, thereby degrading the optical precision of thelens surfaces. For this reason, when an air hole is formed in part ofthe contact portion between the first and second lenses to cause thespace to communicate with the outer air, the temperature and pressuredifferences between the outer air and the space of the circumferentiallycemented lens unit can be minimized. A change in shape of the lenssurface with an abrupt change in outer temperature can be minimized.Therefore, degradation of the optical precision of the lens surface,which is caused by the change in temperature, can be prevented.

In addition, the first and second lenses having the same circumferentialshape can be preferably easily aligned.

In general, a lens is manufactured by machining with reference to theoptical axis. The deviation amount of transmitted light of each singlelens is not so large (the deviation amount of transmitted light isdefined as a deviation angle between the direction of a transmitted beamand the direction of a beam incident on a lens). For this reason, whenthe first and second lenses having the same circumferential shape arecemented while their circumferences are aligned with each other, theresultant lens unit is free from decentering caused by a fixing toolsuch as a lens holder. Therefore, a total deviation amount is only a sumof the deviation amounts of transmitted light of the respective singlelenses. More specifically, even if the peripheral portions of the firstand second lenses are cemented without performing centering, the totaldeviation amount of the cemented lens unit is generally smaller than 30minutes. This value sufficiently satisfies conditions under which alarge deviation amount of transmitted light does not pose any problem,as in an eyepiece or part of a zoom lens.

In particular, when a concave lens is cemented to a lens brought intocontact with the peripheral portion of the concave lens in a minimumdeviation amount of transmitted light, i.e., the lenses are cementedwhile being centered, a cemented lens having a much higher precisionthan that of a conventional cemented lens is provided. For example,according to the present invention, a lens unit satisfying condition E₀<1.5 minutes (where E₀ is the total deviation amount of transmittedlight of the cemented lens) can be easily obtained. The lens unit havingthis value can be applied to an optical product requiring a considerablyhigh precision.

If a deviation amount of transmitted light of one of the lensesconstituting a cemented lens is defined as E₁, any lens unit satisfyingcondition E₁ <E₀ can have a total deviation amount of transmitted lightto be E₀ or less even if the deviation amount of transmitted light ofthe other lens is larger than E₀.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C show a circumferentially cemented lens unit according toan embodiment of the present invention, where FIG. 1A is a sectionalview of a portion including a cemented portion, FIG. 1B is a plan viewof the lens unit, and FIG. 1C is a sectional view including an air holeportion;

FIGS. 2A and 2B show a modification of the lens unit in FIGS. 1A to 1C,wherein lenses are cemented along the entire circumferences thereofwithout forming an air hole, thereby constituting a circumferentiallycemented lens unit;

FIG. 3 is a sectional view showing a circumferentially cemented lensunit according to another embodiment of the present invention;

FIG. 4 is a sectional view showing a telephoto lens according to stillanother embodiment of the present invention;

FIG. 5 is a sectional view showing a conventional example; and

FIG. 6 is a sectional view showing another conventional example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A to 1C are views showing the structure of a circumferentiallycemented lens unit according to an embodiment of the present invention,where FIG. 1A is a sectional view taken along an optical axis 3 of thelens unit and a line A not passing through an air hole 6 in FIG. 1B,FIG. 1B is a plan view of the lens unit, and FIG. 1C is a sectional viewtaken along the optical axis 3 of the lens unit and a line B passingthrough the air hole 6.

The circumferentially cemented lens unit in FIGS. 1A to 1C comprises acemented lens 1 constituted by a single flat positive meniscus lens anda double concave negative meniscus lens, and a single lens 2 having oneconcave surface. The cemented lens 1 and the single lens 2 are cementedat their circumferences to constitute the circumferentially cementedlens unit. The peripheral portion of the single lens 2 having theconcave surface is chamfered to form an outwardly open groove 5 when thesingle lens 2 is brought into contact with the cemented lens 1. All thelenses constituting the circumferentially cemented lens unit have thesame diameter.

In this embodiment, as shown in the sectional view of FIG. 1A, thecemented lens 1 is abutted against and combined with the single lens 2having the concave surface, an adhesive 4 is filled in the groove 5formed by the chamfered portion, and the cemented lens 1 and the singlelens 4 are fixed.

In addition, according to this embodiment, an air hole 6 is formed tocause the outer air to communicate with a space between the cementedlens 1 and the single lens 2 having the concave surface, as shown inFIGS. 1B and 1C. Even if the temperature outside the cemented lensabruptly changes, a change in shape of the lens surface is minimized. Noadhesive 4 is present in a groove 5 portion corresponding to the airhole 6.

The adhesive 4 can have an adhesion strength suitable for theapplication purpose and preferably consists of an ultraviolet curableresin. In this case, additional investments are not required, andoperators need not learn new knowledge. Therefore, the use of theultraviolet curable resin is economically advantageous.

The adhesive can have a strength sufficient to temporarily hold thelenses in the cemented lens when the lenses are finally inserted into alens holder and held by a press ring or the like. However, a cementedlens itself is used as a finished product, an adhesive having a highadhesion power can be used.

In addition, when a lens holder is used as part of a lens, an adhesivefor temporarily cementing lenses and an adhesive having a high adhesionpower can be selectively used depending on different applicationlocations.

In this embodiment, the peripheral portions of the lenses are adhered ata plurality of locations angularly spaced apart from each other so as toform the air hole. However, these lenses may be cemented to each otheralong the entire circumferences thereof, and this modification is shownin FIGS. 2A and 2B. The same reference numerals as in FIGS. 1A and 1Bdenote the same parts in FIGS. 2A and 2B, and a detailed descriptionthereof will be omitted.

FIG. 3 shows another embodiment exemplifying a method of fixing lenses.The respective lenses are chamfered to form grooves open outward whenthey are combined with each other. The lenses are combined and cementedwith each other such that the peripheral portions of the concavesurfaces of lenses 8 are brought into contact with each other. Anadhesive 4 is filled in the grooves open outward when they are combinedwith each other, and the circumference of the resultant lens unit isreinforced with an adhesive tape 7.

Although a small projection is formed on the circumference of theresultant lens by the adhesive tape, a sufficiently high strength whichallows insertion of the lens into an optical system can be obtainedwithout using a lens holder, thereby reducing the labor for insertingthe lens into the lens holder. At the same time, the number of partsused in assembly can be reduced, and the management and material costscan be reduced.

FIG. 4 is a sectional view showing still another embodiment in which thepresent invention is applied to a camera telephoto lens. A telephotolens includes lenses L1 to L7 housed in a barrel 10. The lenses L1 to L3are held in a first holding member 11, the lenses L4 to L6 are held in asecond holding member 12, and the lens L7 is held in a third holdingmember 13. The second holding member 12 is mounted on a movable stage14. The lenses L4 to L6 are axially moved with movement of the movablestage 14, thereby preforming focusing.

In this telephoto lens, the lenses L4 to L6 constitute acircumferentially cemented lens unit. The lenses L4 and L5 constitute acemented lens. The lens L6 is cemented with the cemented lens in thesame manner as in the above embodiments. More specifically, the diameterof each of the lenses L5 and L6 is 37 mm, and these lenses are incontact with each other at a position corresponding to the diameter of34.44 mm. These lenses are cemented to each other at a position radiallyoutward from this contact portion. In assembly of the telephoto lens,the lenses L4 to L6 are cemented as the circumferentially cemented lensunit in advance. This lens unit is then inserted into the holding member12 and is fixed with a press ring 23. Reference numerals 20 to 22 denotepress rings of the lenses L1 to L3.

This circumferentially cemented lens unit may have an air hole as in theone shown in FIGS. 1A to 1C, or the one in which the entirecircumferences are cemented to each other, as shown in FIGS. 2A and 2B.The circumferentially cemented lens unit may be arranged such that anadhesive tape may be applied to the circumference thereof to reinforcethe lens unit, as shown in FIG. 3.

As described above, in a cemented lens according to the presentinvention, lenses can be simply fixed with a higher centering precisionthan that in conventional decentering adjustment using a lens holder.Therefore, productivity of the circumferentially cemented lenses of thepresent invention can be improved.

A cemented lens having a high optical precision can be obtained becausedecentering caused by fixing first and second lenses in a lens holdercan be eliminated.

In addition, when lenses are finely adjusted and cemented, a cementedlens having considerably high performance can be obtained. Even if theselenses are cemented without any fine adjustment, the resultant lens canbe sufficiently used depending on application conditions.

From the viewpoint of lens group assembly, the respective lens groupsmust be conventionally managed at the time of assembly. However, in thecemented lens of the present invention, the lenses can be regarded as alens unit. For this reason, cumbersome operations need not be performedat the time of assembly and adjustment in view of part management. Atthe same time, the management cost can be reduced.

The material cost can also be reduced because the lens holder can beomitted. From the viewpoint of product design, a spatial margin in anoptical system can be increased, so that a new mechanism may be arrangedin an extra space. Alternatively, the lens product can be made compactor the product size can be reduced.

In addition, in a cemented lens of the present invention, an air holecommunicating with the internal space between the lenses can be formedutilizing a cementing portion between the lenses. A change in lenssurface caused by a change in temperature can be minimized.

What is claimed is:
 1. A circumferentially cemented lens unit comprisingfirst and second lenses opposed to each other with a space therebetweenon an optical axis thereof, wherein a contact portion of an opposingsurface of one of said first and second lenses is disposed inward fromthe periphery of said one lens with respect to the optical axis and isin contact with a curved opposing optical surface of the other of saidfirst and second lenses near the periphery thereof, and at least one ofthe opposing surfaces of the first and second lenses is a concavesurface, and said first and second lenses are cemented to each other ata portion outward from said contact portion with respect to the opticalaxis.
 2. A lens unit according to claim 1, wherein said first and secondlenses are continuously cemented along entire circumferences thereof. 3.A lens unit according to claim 1, wherein said first and second lensesare cemented at a plurality of locations spaced apart from each otheralong a circumferential direction.
 4. A lens unit according to claim 1,wherein a peripheral portion of at least one of said first and secondlenses is chamfered to form a groove open radially outward from saidcontact portion, said groove being filled with an adhesive.
 5. A lensunit according to claim 1, wherein an air hole for causing the space tocommunicate with outer air is formed at said contact portion betweensaid first and second lenses.
 6. A lens unit according to claim 1,wherein said first and second lenses have the same outer shape.
 7. Alens unit according to claim 1, wherein said first and second lenses arecemented to each other such that a total deviation amount of transmittedlight becomes minimum.
 8. A lens unit according to claim 1, whereincondition E₀ <30 minutes is satisfied, where E₀ is a total deviationamount of transmitted light of said lens unit.
 9. A lens unit accordingto claim 1, wherein condition E₀ <1.5 minutes is satisfied, where E₀ isa total deviation amount of transmitted light of said lens unit.
 10. Alens unit according to claim 1, wherein condition E₁ <E₀ is satisfied,where E₀ is a total deviation amount of transmitted light of said lensunit and E₁ is a deviation amount of transmitted light of one of thelenses constituting said lens unit.
 11. A lens unit according to claim1, wherein said cemented lens unit uses adhesive that essentiallyconsists of an ultraviolet curable resin.
 12. A lens unit according toclaim 1, wherein an adhesive tape is adhered to a circumference of saidlens unit.
 13. A lens system comprising:a plurality of lenses; and alens holder for housing said plurality of lenses; characterized in thatat least two of said plurality of lenses are cemented to form acircumferentially cemented lens unit which includes first and secondlenses opposed to each other with a space therebetween on an opticalaxis thereof, a contact portion of an opposing surface of one of saidfirst and second lenses being disposed inward from the periphery of saidone lens with respect to the optical axis and being in contact with acurved opposing optical surface of the other of said first and secondlenses near the periphery thereof, at least one of the opposing surfacesof the first and second lenses being a concave surface, and said firstand second lenses being cemented to each other at a portion outward fromsaid contact portion with respect to the optical axis.
 14. A lens systemaccording to claim 13, wherein said first and second lenses arecontinuously cemented along entire circumferences thereof.
 15. A lenssystem according to claim 13, wherein said first and second lenses arecemented at a plurality of locations spaced apart from each other alonga circumferential direction.
 16. A lens system according to claim 13,wherein a peripheral portion of at least one of said first and secondlenses is chamfered to form a groove open radially outward from saidcontact portion, said groove being filled with an adhesive.
 17. A lenssystem according to claim 13, wherein an air hole for causing the spaceto communicate with outer air is formed at said contact portion betweensaid first and second lenses.
 18. A lens system according to claim 13,wherein said first and second lenses have the same outer shape.
 19. Alens system according to claim 13, wherein said first and second lensesare cemented to each other such that a total deviation amount oftransmitted light becomes minimum.
 20. A lens system according to claim13, wherein condition E₀ <30 minutes is satisfied, where E₀ is a totaldeviation amount of transmitted light of said lens system.
 21. A lenssystem according to claim 13, wherein condition E₀ <1.5 minutes issatisfied, where E₀ is a total deviation amount of transmitted light ofsaid lens system.
 22. A lens system according to claim 13, whereincondition E₁ <E₀ is satisfied, where E₀ is a total deviation amount oftransmitted light of said lens system and E₁ is a deviation amount oftransmitted light of one of the lenses constituting said lens system.23. A lens system according to claim 13, wherein said cemented lens unituses adhesive that essentially consists of an ultraviolet curable resin.24. A lens system according to claim 13, wherein an adhesive tape isadhered to a circumference of said lens unit.
 25. A lens systemaccording to claim 13, further including a press ring for fixing saidlens unit in said lens holder.
 26. A lens unit according to claim 1,wherein said curved opposing optical surface is a concave surface.
 27. Alens unit according to claim 13, wherein said curved opposing opticalsurface is a concave surface.